External IP Extension Not Working

I have a Talkswitch 84 VSX.

I'm having issues with an external ip extension not working.  It worked before and now it is not working.  I tried to setup another remote ip phone to test and I can't get it to work either.  I have all port forwarding configured properly on our office firewall (as I said it was working before).  I use dynu.net to do dynamic DNS.  I have tried resetting the phone to default, and then setting the TFTP server to the domain that points to our office ip address.

What is the best way to diagnose the issue?  I can ping the domain fine from the remote location and it resolves to the correct IP address.  I checked the firewall settings on the office cable modem, and they are all correct.